1. Field of the Invention
The present invention relates to a glass spraying method and apparatus for forming a glass coating on a metal surface.
2. Description of the Related Art
For example, as a roll for printing or an industrial roll used for papermaking or steelmaking, rolls made of metal have been used widely. Since the metallic rolls are brought into contact with a printing ink exhibiting a strong acidity or alkalinity, a cleaning solution, paper or metallic products, etc., the deterioration in quality of the surfaces of the metallic rolls is accelerated with time due to corrosion, etc. The rolls are, in many cases, used under hostile conditions such as high temperature and high humidity and have a problem that they are inferior in durability since the surface thereof is deteriorated rapidly due to the contamination on the roll surface.
Furthermore, a roll for conveying goods or articles or a guiding roll have preferably electrically insulating property since it has been recently used with electronic equipment, and it is preferable that contaminant hardly stick to the roll surface or can be removed easily from the surface if the contaminant sticks to the surface.
To solve the above-mentioned technical problems, a roll having a glass coating on the surface thereof has been proposed. The inventors has disclosed a roll base having a glass surface and a method for manufacturing the same in Japanese Kokai No. 4-99259 and a spray gun for a spraying apparatus useful for forming a glass coating in Japanese Patent No.2562801. A technology to form a glass coating sprayed on the surface of a metallic roll is disclosed in Kokai No. 64-13324.
It is needless to say that controlling the temperature of a metallic base is important upon forming a glass coating by spraying a glass material onto the metallic base. According to the research of the inventors, it is important to heat the metallic base, upon spraying, to a high temperature of about 600 to 1000xc2x0 C. which is above a softening point of a glass, to form a glass coat of a high quality. However, apart from a small roll, it requires a long period of time and an enormous amount of energy to entirely heat a huge metallic base to a temperature of 600 to 1000xc2x0 C., which is commonly used for the previously mentioned industrial roll whose diameter and length sometimes amount to 200 to 300 mm and 1000 mm or more, respectively.
On the other hand, when a metallic base is kept under such a high temperature, a base made of an iron material such as SS41 may be deteriorated or a serious problem in forming a glass coating may occur in a base made of a martensitic stainless steel due to a significant change in thermal expansion coefficient.
Furthermore, when a glass coating having been formed by spraying is subjected to a high temperature for a long time (about 60 min or more depending on the components), the glass layer is recrystallized resulting in the deterioration of the glass.
When a glass material of a high temperature is directly sprayed onto the surface of a metallic base, the base surface is oxidized to form a oxide film (so-called black rust) resulting in a lowered adhesion strength of the glass coating to the metallic base. To prevent the formation of the oxide film, it has been proposed to provide, for example, an under-coat layer of stainless in above mentioned Kokai No. 4-99259 and an under-coat layer of an alloy of such as Nixe2x80x94Cr in Kokai No. 64-13324.
However, when the metallic under-coat of stainless or an alloy is formed, air intervening between the surface of the metallic base and the under-coat is heated by high temperature upon spraying the glass material and may be trapped as air bubbles in the glass coating. The air bubbles thus formed have a diameter of 10-44 xcexcm at the smallest and of 66 xcexcm at the largest and appear on the surface of the glass coating after polishing as minute pin holes resulting in lowered commercial value of the product.
The adhesion between the metallic under coat layer of stainless or an alloy and the glass coating is not good. In addition, the adhesion rate of stainless or an alloy upon forming the metallic under coat layer by spraying, is about 60 to 70% and the availability of the material is not good.
The main object of the present invention, which has been proposed based on the above circumstances, is to spray a glass material onto a metallic base effectively and efficiently. The present invention also provides a glass spraying method wherein the generation of the pin holes in the glass coating is prevented and the strength, adhesion strength and the appearance of the resulting glass coating is largely improved. Furthermore, the present invention provides an apparatus useful for spraying glass onto a metallic roll base, particularly a large metallic roll base.
To achieve the above-mentioned object, an invention of a glass spraying method described in claim 1 is proposed. The invention of claim 1 comprises a preheating step to heat a metallic base entirely, a pre-spraying step to form a glass pre-coating by pre-spraying a glass material onto the surface of the preheated base, and a main-spraying step to form a glass coating by additionally heating the glass pre-coating formed in the previous step and main-spraying a like glass material onto the surface thereof.
The invention of claim 2 relates to a glass spraying method according to claim 1, further comprising a post-heating step following the main-spraying step wherein the glass coating which has been formed by main-spraying is heated.
The invention of claim 3 relates to a glass spraying method according to claim 1 or 2, wherein a glass pre-coating having a thickness of 50 xcexcm to 200 xcexcm is formed in the pre-spraying step.
The invention of claim 4 relates to a glass spraying method according to claim 1, wherein the metallic base is a metallic roll base which is held to be rotatable in the circumferential direction, so that said steps are performed while rotating the metallic roll base.
The invention of claim 5 relates to a glass spraying method according to claim 4, wherein the pre-spraying step, the main-spraying step and the post-heating step are performed by an apparatus which moves continuously along the axial direction of the metallic roll base.
The invention of claim 6 relates to an glass spraying apparatus comprising, a rotating and holding apparatus which holds a metallic roll base and rotates the same in the circumferential direction thereof, an entire heating apparatus disposed in the axial direction of the roll base to heat the roll base entirely, a pre-spraying apparatus which moves along the axial direction of the roll base and sprays a glass material onto the surface of the roll base in sequence to form a glass pre-coating, an additional heating apparatus which moves along the axial direction of the roll base and heats additionally the glass pre-coating locally and continuously, and a main-spraying apparatus disposed on the rear side of the additional heating apparatus in the direction of travel thereof, which sprays a glass material in sequence onto the surface of the glass pre-coating which has been additionally heated.
The invention of claim 7 relates to an glass spraying apparatus according to claim 6, further comprising a post-heating apparatus disposed on the rear side of the main-spraying apparatus in the direction of travel thereof, which heats the glass-coating which has been subjected to the main-spraying.
The invention of claim 8 relates to an glass spraying apparatus according to claim 6 or 7, wherein the additional heating apparatus, the main-spraying apparatus, and the post-heating apparatus are arranged so that they are moved together at one time.